Nanotechnology-based Treatment to Produce More Clean Energy, Less Landfill Waste
09/06/2018
By Edwin L. Aguirre
Each year, the United States produces more than 12 million tons of sludge – the thick mixture of solid and liquid matter left over from processing wastewater and raw sewage in treatment plants. And it costs taxpayers roughly $2 billion annually to safely handle, treat and dispose of the unwanted byproduct.
Massachusetts alone produces 620,000 tons of sludge every year and uses up to 10 million watt-hours of energy to process the waste.
“There is a lot of wastewater sludge in the country, and it is very expensive to deal with it,” says Asst. Prof. Onur G. Apul of the Department of Civil and Environmental Engineering.
According to Apul, only half of the sludge gets converted to biogas fuel (mainly methane); the rest is disposed of in incinerators and landfills.
Apul and a team of UML researchers have invented a more environmentally friendly, efficient and potentially revenue-generating alternative. They have developed a single-step thermal treatment technique that increases biogas production by up to 300 percent while reducing the amount of residual sludge that goes into landfills by 20 to 30 percent.
“The technology – which uses microwave radiation together with nanofiber additives – is new, and we applied for a patent for it this year through the university’s Office of Technology Commercialization,” says Apul. “We can contribute to clean-energy production and also help treatment plants to discharge less waste because more sludge can now be converted to biogas.”
Aside from Apul, the research team includes Prof. Xiaoqi (Jackie) Zhang of the Department of Civil and Environmental Engineering, Ph.D. student Arsalan Khalid and undergraduate student Ritchie LaFaille.
The project, which is funded with a one-year, $65,000 grant from the Massachusetts Clean Energy Center’s Catalyst Program, is a collaboration with the Lowell Regional Wastewater Utility, which serves the city of Lowell as well as the surrounding towns of Chelmsford, Dracut, Tewksbury and Tyngsboro.
“Using our technology, a city with a population of 75,000, for example, would save about $210,000 a year in sludge transportation and disposal, as well as earn an additional $74,000 a year from biogas revenue,” says Apul.
An Innovative Solution
In a treatment plant, the wastewater is first treated with biological methods, using aerobic microorganisms in a process called activated sludge. Gravity then separates water from the waste sludge, and the water is disinfected to kill any pathogens. The treated, clean water is typically discharged to a nearby river or lake.
The sludge is sent for further processing into a digester, a huge tank where anaerobic bacteria break down the sludge’s organic matter and convert it to methane. About half of the sludge produced is digested by the bacteria, and the remaining sludge goes to a landfill.
The researchers’ method calls for heating the sludge with microwave radiation (similar to what is used in kitchen microwaves) before it goes to the digester. The radiation breaks down the cell walls of microbes, making them more soluble in water and decreasing clumping of the cells. The hydrolyzed sludge then goes to the digester.
To make the process more efficient, the team adds carbon-based dielectric (nonconducting) nanofibers to the sludge, dramatically speeding up the microwave heating. The high temperature increases the sludge’s concentration of soluble organic compounds that get converted to methane in the digester.
“Aside from improving the digester’s methane production by three times, our method offers other advantages, including low-energy requirement and safe and easy-to-deploy microwave irradiation, and no additional chemicals are needed,” says Apul.
He says the microwave-nanofiber treatment method may reach, and even surpass, the efficiencies of other commercial techniques that are currently used to hydrolyze sludge – with ultrasound, ozone, alkaline/acid chemicals and thermal (non-microwave) treatment, among others.
Revenue-making Potential
At treatment plants, the methane produced is captured and used to heat the digesters and keep the tanks at the appropriate temperature. Any excess methane is usually burned off into the atmosphere.
The high-tech Deer Island Wastewater Treatment Plant on Boston Harbor, operated by the Massachusetts Water Resources Authority (MWRA), does not burn off surplus methane, according to Apul. “This one-of-a-kind facility in the state has a system where excess methane is piped to boilers to heat buildings onsite and generate about 3 million watts of electricity via steam turbine generators,” he says. “This saves MWRA approximately $15 million annually in heating and electricity costs.”
The team’s vision is to turn the business of wastewater treatment into a self-sustaining, and even profit-making, enterprise by selling the surplus methane-generated electricity to the local power grid. Aside from municipal sewage treatment plants, other sources of sludge that contain high organic carbon include the sugar industry, manure from farms, agricultural discharges and the food-waste industry.
“There is a huge revenue potential for our microwave-nanofiber treatment method,” says Apul. “Depending on the market needs and technology readiness, our team can start a spinoff company or license the technology to an existing microwave application company.”